The present invention is directed generally to photoresponsive imaging members, and more specifically, to photoconductive imaging members comprised of certain perylene photogenerating pigments. In embodiments, the present invention is directed to layered photoconductive imaging members comprised of certain benzimidazole perylenes, reference U.S. Pat. Nos. 4,587,189 and 5,225,307, the disclosures of which are totally incorporated herein by reference, and wherein perylenes with improved photosensitivity and dispersion quality can be generated by the contacting thereof with an organic solvent, and more specifically, with cyclohexane. Embodiments of the present invention are directed to imaging members comprised of a supporting substrate, a photogenerating layer comprised of the prepared perylene, and in contact therewith, and a charge, especially hole, transport layer. Imaging members with the photogenerating pigments of the present invention are sensitive to wavelengths of from about 400 to about 800 nanometers, that is throughout the visible and the near IR region of the light spectrum.
Generally, layered photoresponsive imaging members are described in a number of U.S. patents, such as U.S. Pat. No. 4,265,900, the disclosure of which is totally incorporated herein by reference, wherein there is illustrated an imaging member comprised of a photogenerating layer, and an aryl amine hole transport layer. Examples of photogenerating layer components include trigonal selenium, metal phthalocyanines, vanadyl phthalocyanines, and metal free phthalocyanines.
The use of selected perylene pigments as photoconductive substances is also known. There is thus described in Hoechst European Patent Publication 0040402, DE3019326, filed May 21, 1980, the use of N,N'-disubstituted perylene-3,4,9,10-tetracarboxyldiimide pigments as photoconductive substances. Specifically, there is, for example, disclosed in this publication N,N'-bis(3-methoxypropyl)perylene-3,4,9,10-tetracarboxyldiimide dual layered negatively charged photoreceptors with improved spectral response in the wavelength region of 400 to 700 nanometers. There are disclosed in U.S. Pat. No. 3,871,882 photoconductive substances comprised of specific perylene-3,4,9,10-tetracarboxylic acid derivative dyestuffs. In accordance with the teachings of this patent, the photoconductive layer is preferably formed by vapor depositing a dyestuff component in a vacuum. Also, there are specifically disclosed in this patent dual layer photoreceptors with perylene-3,4,9,10-tetracarboxylic acid diimide derivatives, which have spectral response in the wavelength region of from 400 to 600 nanometers.
Moreover, there are disclosed in U.S. Pat. No. 4,419,427 electrographic recording mediums with a photoconductive double layer comprised of a first layer containing charge carrier perylene diimide dyes, and a second layer with one or more compounds which are charge transporting materials when exposed to light, reference the disclosure in column 2, beginning at line 20.
Various types of perylene pigments, symmetrical as well as unsymmetrical structures, are known and are illustrated herein. Perylenes with these general structures can be prepared by reacting perylene tetracarboxylic acid dianhydride with primary amines or with diamines of aryl, alkyl, pyridyl, or naphthyl compounds. More specifically, the use of photoconductive perylene pigments obtained from perylene-3,4,9,10-tetracarboxylic acid dianhydride as photoconductors is disclosed in U.S. Pat. Nos. 3,871,882 and 3,904,407. The former patent illustrates the use of perylene dianhydride and bisimides in general (R.dbd.H, lower alkyl, like C.sub.1 to C.sub.4, aryl, substituted aryl, arylalkyl, a heterocyclic group or the NHR' group in which R' is phenyl, substituted phenyl or benzoyl) as vacuum evaporated thin charge generation layers (CGLs) in photoconductive devices coated with a charge transporting layer (CTL). The '407 patent, the disclosure of which is totally incorporated herein by reference, illustrates the use of general bisimide compounds (R=alkyl, aryl, alkylaryl, alkoxyl or halogen, or heterocyclic substituent) with preferred pigments being wherein R is chlorophenyl or methoxyphenyl. Perylenes and processes thereof are illustrated in U.S. Pat. Nos. 5,019,473 and 5,225,307, the disclosures of which are totally incorporated herein by reference.
Photoresponsive members contain a uniform layer of charge generator which is comprised of fine particles of photogenerator pigment. Consequently, there have been developed various processes to prepare the fine particles of pigment such that the final coating of the generator layer has a high dispersion quality, and controllable electrical and printing properties. However, photoresponsive elements containing photoconductive perylenes and the processes thereof described in a number of prior art references has certain deficiencies, for example with the processes of U.S. Pat. Nos. 4,587,189 (BZP) and 4,578,333 a vacuum coating process is selected to prepare a submicron thick charge generator layer for the photoresponsive member. This layer is usually thin, lacks substantial mechanical abrasion resistance, and can be easily damaged by rubbing and scratching during the handling processes. This can severely increase the defects in photoresponsive member which later cause printing defects. These and other problems are resolved or minimized with the processes of the present invention.
In another approach, sublimation process is selected to purify perylene pigments to remove detrimental impurities from the crude, assynthesized materials, reference U.S. Pat. No. 5,225,307. Following the purification, a certain polymer binder dispersion of perylene is prepared by milling pigment, binder and solvent in accordance with this conventional approach. The photoresponsive elements prepared from such dispersions suffer a significant loss in photosensitivity. Furthermore, there are various risks resulting from the prolonged milling process, for example, the polymer binder may break up into smaller fragments resulting in unstable dispersions due primarily to the loss of the stabilization effect provided by polymer, and the poor coating uniformity of the charge generator.
Premilling process is also commonly used in particle size reduction processes as practiced in the pigment and paint industries. Typically, the pigment is ground in the dry state with grinding media, such as steel balls, ceramic balls, or glass beads in jar mills, vibratory mills, or attritors. Inorganic salts, such as alkali halides, carbonates, sulfates or phosphates, are also added to the grinding mixture to improve process efficiency. High shear and mechanical impact forces produced in the grinding action break the pigment agglomerates into finer sizes. In U.S. Pat. No. 5,019,473 there is illustrated the use of dry premilling steps for reducing the particle size of perylene pigments prior to using them in preparing charge generator dispersions. Contaminants introduced by salts have to be thoroughly washed and removed, or separated from the pigments, otherwise final electrical and printing performance of the photoresponsive members would be severely adversely affected. These multiple processing steps involve milling, washing separation and drying which severely increase the cost of the manufacturing process and the process variability. Furthermore, certain perylene pigments may have a high adhesion force and thus tend to stick together as large agglomerates, and the dry milling process as described in U.S. Pat. No. 5,019,473 may not be effective in reducing the particle size. There is also a significant loss of material which remains stuck onto the wall of the preparation vessel used in processing the pigment, and a large particle size distribution is also observed.
Acid pasting process for reducing pigment particle size is also known. For example, in U.S. Pat. No. 4,557,868 a phthalocyanine was dissolved in concentrated sulfuric acid to form an acid mixture which was then diluted in water to produce fine particles of phthalocyanine. The resulting fine particles were used to form the charge generator of a photoresponsive member for electrophotographic applications. Many phthalocyanine pigments, such as VOPc, TiOPc, H.sub.2 Pc, and CuPc, have been reportedly treated in this manner. Unfortunately, the reaction of perylene and many pigments with acids result in chemical degradation and the formation of sulfonated products which finally leads to poor electrical charging properties in the charge generator layer. As a result, the acid pasting process is not particularly suitable for preparing high quality, photoconductive perylene pigments.
The aforementioned disadvantages are avoided and/or minimized with the processes of the present invention. More specifically, extensive research was accomplished with the primary objectives of simplifying the particle size reduction process for perylene pigments, and which processes are particularly useful in forming photoresponsive imaging members, and wherein the final electrical and printing characteristics are improved. In addition to small particle size, it is believed that narrow size distribution, and correct aspect ratio, the particle morphology of pigment can affect the properties of generator layer and hence the performance of photoresponsive members. For practical electrophotographic applications, a higher photosensitivity of photoresponsive members is desirable for achieving higher printing speed and better dispersion quality can significantly improve the electrical and printing quality. Less pigment agglomeration correspondingly minimizes the number of charge deficiency spots which can cause printing defects. More specifically, the perylene pigments of benzimidazole perylene pigments as synthesized and purified in U.S. Pat. No. 5,225,307 wherein perylene pigment particles of very large particle sizes, large size distribution and high aspect ratio were obtained from the sublimation process possess concerns relating to coating uniformity and the photosensitivity property of the final photoresponsive members. In accordance with embodiments of the present invention, large particles, up to one millimeter in length, of sublimed benzimidazole perylenes are subjected to milling in an organic solvent, preferably cyclohexane, to reduce to a smaller size &lt;0.15 micron in length, and a low aspect ratio (length/width) equal or smaller than 3. The finely divided, cyclohexane treated perylene obtained can be selected for generating a charge generator dispersion. The perylene obtained from the cyclohexane treatment can be used directly in coating the charge generator. Alternatively, for achieving improved adhesion and mechanical properties in the final generator layer, the cyclohexane treated perylene can be redispersed in a polymer/solvent mixture to form a generator dispersion which is used to coat the generator layer. These dispersions permit significant improvements in coating uniformity of the charge generator layer and improvements in photosensitivity. It is believed that smaller size, low aspect ratio and more uniform particle size distribution in cyclohexane treated perylene are advantageous in enabling excellent dispersion quality of the generator layer and a photosensitivity improvement. Furthermore, the processes of the present invention are devoid of many of the difficulties encountered in prior art methods, such as contamination by salts, tedious washing and drying steps for separating pigment after the particle reduction process and the like. Once the perylene is processed in the cyclohexane, the fine particles can be conveniently redispersed in a polymer/solvent mixture to form the generator dispersion without a prolonged milling process and, therefore, there is eliminated the need of breaking the polymer into small fragments which would reduce the polymer's stabilizing power for the fine particles. Ideally, a photoconductive pigment used for light lens imaging, particularly for color photocopying, should have uniform spectral response, that is be panchromatic throughout the visible spectrum from 400 to 800 nanometers. More specifically, there is a need for the generation of perylenes with improved sensitivity thereby enabling their use in layered photoconductive imaging members that can be selected for high speed printing, for example 100 or more copies per minute, xerographic imaging and printing systems. Additionally, there is a need for photoconductive materials that enable imaging members with enhanced photosensitivity in the red and infrared wavelength regions of the light spectrum, enabling the resulting imaging members thereof to be selected for LED xerographic imaging processes and printers, and diode laser printer and imaging apparatuses.